Your last statement is correct. There are a limited number of trainers, and the training is quite intense. Any online teaching scenario has to limit the number of students because there are still people who must read the material the students write and provide feedback. That takes time and thought. Increasing the number of students would have negative consequences on the training and development of all the students involved.
The only situation in which this is not the case is if the teaching/training is a fully automatic system. In that case, the feedback is limited in scope. It may work for some test preparation or fairly simple on-going training situations, but that is about it. Further, if a student doesn't understand the explanation or questions it, there is very little recourse and no opportunity for discussion unless such training is a supplement to something else that does have human interaction.
Orange Blossom